1. Technical Field
The invention is related to a plasma reactor for processing a workpiece such as a semiconductor wafer with a process employing an etch selectivity-enhancing precursor material such as polymer precursor gases.
2. Background Art
High density RF plasma reactors for etching contact openings through silicon dioxide layers to underlying polysilicon conductor layers and/or to the silicon substrate of a semiconductor wafer are disclosed in the above-referenced application by Collins et al. Ideally, such a reactor carries out an etch process which quickly etches the overlying silicon dioxide layer wherever a contact opening is to be formed, but stops wherever and as soon as the underlying polysilicon or silicon material (or other non-oxygen-containing material such as silicon nitride) is exposed, so that the process has a high oxide-to-silicon etch selectivity. Such reactors typically include a vacuum chamber, a wafer support within the chamber, process gas flow inlets to the chamber, a plasma source coil adjacent the chamber connected to an RF power source usually furnishing plasma source power and another RF power source connected to the wafer support usually to furnish plasma bias power. For a silicon oxide etch process, a process gas including an etchant such as a fluorine-containing substance is introduced into the chamber. The fluorine in the process gas freely dissociates under typical conditions so much that the etch process attacks not only the silicon oxide layer through which contact openings are to be etched, but also attacks the underlying polysilicon or silicon material as soon as it is exposed by the etch process. Thus, a typical etch process carried out by such a reactor is not the ideal process desired and has a lower oxide-to-silicon etch selectivity. As employed in this specification, the term xe2x80x9cetch selectivityxe2x80x9d refers to the ratio between the etch rates of two different materials, such as silicon dioxide and silicon (either crystalline silicon or polycrystalline silicon hereinafter referred to as xe2x80x9cpolysiliconxe2x80x9d). A low etch selectivity can cause punch through. In etching shallow contact openings to intermediate polysilicon layers while simultaneously etching deep contact openings to the underlying silicon substrate, the etch process first reaches and will punch through the intermediate polysilicon layer before reaching the silicon substrate. A very high oxide-to-silicon etch selectivity is necessary to prevent the punchthrough, depending upon the ratio between the depths of the silicon substrate and the intermediate polysilicon layer through the silicon oxide. For example, if (a) the deep contact opening through the oxide to the substrate is 1.0 micron deep and is to be 50% overetched, (b) the intermediate polysilicon layer is 0.4 microns deep (below the top of the oxide layer) and (c) if not more than 0.01 microns of the intermediate polysilicon layer are to be removed (to avoid punch-through), then an oxide-to-silicon etch selectivity of at least 110:1 is required.
It is known that oxide-to-silicon etch selectivity is enhanced by a polymer film which forms more readily over silicon and polysilicon or other non-oxygen-containing layers than over silicon dioxide or other oxygen-containing layers. In order to form such a selectivity-enhancing polymer film, the fluorine-containing substance in the process gas is a fluorocarbon or a fluoro-hydrocarbon. Some of the fluorine in the process gas is consumed in chemically etching the silicon dioxide layer on the wafer. Another portion of the fluorine reacts with other species including carbon contained in the process gas to form a polymer on the surface of the wafer. This polymer forms more rapidly and strongly on any exposed silicon and polysilicon surfaces (or other non-oxygen-containing surfaces) than on silicon dioxide (or other oxygen-containing surfaces), thus protecting the silicon and polysilicon from the etchant and enhancing etch selectivity. Etch selectivity is further improved by improving the strength of the polymer formed on polysilicon surfaces. The polymer is strengthened by increasing the proportion of carbon in the polymer relative to fluorine, which can be accomplished by decreasing the amount of free fluorine in the plasma. For this purpose, a fluorine scavenger, such as a silicon piece, can be provided in the reactor chamber and heated, to avoid being covered with polymer and additionally to permit silicon ions, radicals and/or neutral species to be removed therefrom and taken into the plasma. The silicon atoms removed from the scavenger combine with some of the free fluorine in the plasma, thereby reducing the amount of fluorine available to polymerize and increasing the proportion of carbon in the polymer formed on the wafer.
While the use of a fluorine scavenger such as a heated silicon piece inside the reactor chamber enhances etch selectivity by strengthening the polymer formed on the wafer, even the etch selectivity so enhanced can be relatively inadequate for a particular application such as the simultaneous etching of contact holes of very different depths. Therefore, it would be desireable to increase the polymer strength beyond that achieved by the improved scavenging technique described above.
Another problem is that the rate of removal of silicon from the scavenger piece required to achieve a substantial increase in polymer strength is so great that the silicon piece is rapidly consumed and the consequent need for its replacement exacts a price in loss of productivity and increased cost. Typically the scavenger piece is a piece of silicon in the reactor chamber ceiling or wall or a piece of silicon near the reactor chamber ceiling. The rate of removal of silicon therefrom is enhanced by applying an RF bias potential to the silicon piece while its temperature is carefully controlled to a prevent polymer deposition thereon and to control the rate of silicon removal therefrom. As disclosed in the above-referenced U.S. application Ser. No. 08/543,067, silicon is added into the plasma by a combination of applied RF bias and heating of the scavenger piece. The temperature control apparatus is integrated with the silicon piece so that replacement of the silicon piece (e.g., a silicon ceiling) is relatively expensive. In U.S. application Ser. No. 08/597,577 referenced above, an all-silicon reactor chamber is disclosed in which the walls and ceiling are silicon, and any fluorine scavenging is done by consuming the silicon ceiling or walls, requiring their replacement at periodic intervals with a concomitant increase in cost of operation and decrease in productivity. Thus, not only is it desireable to increase the polymer strength but it is also desireable to decrease the rate at which silicon must be removed from the scavenger to achieve a desired etch selectivity.
It is a discovery of the invention that by raising the temperature of a polymer hardening precursor material such as silicon inside the reactor chamber beyond that required to merely scavenge fluorinexe2x80x94i.e., into a higher temperature range, a different more durable polymer is formed over exposed silicon and polysilicon surfaces which is more resistant to etching than has been possible heretofore by merely scavenging fluorine. In this respect, the term xe2x80x9cpolymer hardening precursorxe2x80x9d refers to a material in the chamber which, when its temperature is increased, increases the resistance to etching of the polymer formed on the wafer in accordance with the temperature increase. The polymer formed by holding the polymer-hardening precursor material at the higher temperature range is more durable than polymer formed otherwise, and protects the silicon and polysilicon surfaces so much better that oxide-to-silicon etch selectivity is substantially enhanced over that attained heretofore. Material from the heated polymer-hardening precursor (e.g., silicon) piece participates favorably in the polymerization process by changing the process gas content ratios of carbon-to-fluorine, hydrogen-to-fluorine and carbon-to-hydrogen as a function of its increased temperature, so that the resulting polymer is substantially strengthened. As the polymer-hardening precursor piece in the reactor chamber is heated above the polymerization temperature (the temperature below which polymer precursor materials can condense onto the surface) and into the higher temperature range, the etch selectivity increases with the temperature increase. Thus, a general method of the invention is to provide a polymer-hardening precursor piece (such as silicon, carbon, silicon carbide or silicon nitride, but preferably silicon) within the reactor chamber during an etch process with a fluoro-carbon or fluoro-hydrocarbon gas, and to heat the polymer-hardening precursor piece above the polymerization temperature sufficiently (i.e., into the higher temperature range) to achieve a desired increase in oxide-to-silicon etch selectivity beyond that heretofore attained.
In accordance with an alternative embodiment of the invention, it is a discovery of the invention that the temperature of the polymer-hardening precursor material can be increased even further into a maximum temperature range at which the hardness of the resulting polymer is even greater. In some cases, this is indicated by a shiny appearance of the polymer. It is believed that in this maximum temperature range, material from the polymer-hardening precursor piece enters into the polymer to achieve the extremely hard polymer. For example, if the polymer-hardening precursor material is silicon and is held at this maximum temperature range, then the resulting polymer on the wafer contains silicon.
The higher temperature range of the first embodiment and the maximum temperature range of the second embodiment depend upon the RF bias applied to the polymer-hardening precursor piece. In the absence of an external applied RF bias or potential on a polymer-hardening precursor piece of crystalline silicon, the higher temperature range was about 100xc2x0 C. to about 220xc2x0 C. while the maximum temperature range was above 220xc2x0 C. and preferably between 300xc2x0 C. and 700xc2x0 C. However, any directly or indirectly applied RF bias power quickly shifts such temperature range downwardly.
The polymer-hardening precursor (e.g., silicon) piece may be an integral part of the reactor chamber walls and/or ceiling. However, it is preferably a separate, expendable and quickly removable piece, and the heating/cooling apparatus may be of any suitable type including apparatus which conductively or remotely heats the polymer-hardening precursor piece. Alternatively, if plasma heating of the polymer-hardening precursor piece is sufficient, the desired effect is achieved by refraining from cooling the polymer-hardening precursor piece so as to maintain it at least in the higher temperature range (i.e., above the polymerization temperature). In this alternative mode, the requisite heating of the polymer-hardening precursor piece is accomplished by exploiting plasma heating in lieu of conduction heating apparatus mechanically coupled to the polymer-hardening precursor piece.
In accordance with a preferred embodiment of the invention, no heating apparatus is directly or mechanically coupled to the polymer-hardening precursor piece, thereby permitting the piece to be cheaply fabricated and quickly removable from the reactor chamber. In this form, the polymer-hardening precursor piece is a simply-shaped expendable item in the reactor chamber separate from the chamber structural features such as the wall and ceiling, and has no mechanical features for coupling to other apparatus such as heating devices. Preferably, the heating apparatus heats the polymer-hardening precursor piece by radiation or induction rather than conduction to avoid mechanical coupling therewith, so as to be unaffected by removal and replacement of the expendable piece and so as to be free from temperature sensitivity to mechanical connections. Also, where cooling of the silicon piece is required, it is preferred to employ radiant cooling to avoid mechanical coupling. Similarly, temperature control is achieved by remotely (e.g., by re-radiation as with an optical pyrometric temperature probe or by stimulated emission as with a fluoro-optical temperature probe) sensing the silicon piece temperature, so that no temperature sensor apparatus is mechanically coupled to the silicon piece. Thus, a preferred embodiment employs radiant (or inductive) heating, radiant cooling, and remote temperature sensing of the polymer-hardening precursor piece to eliminate sensitivity of the temperature control to mechanical contact.
Remote temperature sensing of the polymer-hardening precursor piece can be performed using devices such as an optical pyrometer or a fluoro-optic probe. An advantage of the latter is that it is independent of the thermal emissivity of the material being measured.
In one aspect of the invention, the silicon piece functions not only as a polymer hardening precursor material but also as a shield between the heat source and the plasma source region preventing the heat source (e.g., a radiant or inductive heater) from generating plasma. It also shields the heat source (or its window) from exposure to the plasma or its corrosive effects.
In a preferred implementation, the expendable polymer-hardening precursor piece is a planar silicon annulus or base plate extending radially outwardly from a circumferential periphery of the wafer support or wafer chuck toward the chamber sidewall. (Further, if desired the silicon base plate may serve as a heat shield to protect from the plasma an underlying ceramic clamp (or electrostatic chuck) holding the wafer on the wafer pedestal used in certain types of plasma reactors.) In the preferred implementation, the silicon base plate is heated through inductive heating by an underlying inductor, although any other suitable remote heating technique may be employed, such as infrared radiation heating. For this purpose, silicon material of an appropriate resistivity is selected is for the heated silicon piece to assure efficient inductive heating thereof by the underlying inductor and at least nearly complete absorption of the induction field so that the silicon piece functions as a plasma shield as well as a heat shield. The temperature control system monitors the silicon base plate temperature using a radiant temperature sensor facing the silicon piece through a radiantly transparent window, or through a window which is at least nearly transparent at a wavelength range within which the temperature sensor responds. In one implementation, the radiantly transmissive window is quartz, the sensor is an optical pyrometer, and a small black-body radiator piece or gray-body radiator piece, such as a small piece of silicon nitride, is bonded to a location on the silicon base plate viewed by the temperature sensor to enhance the sensor""s performance.
If the present invention is employed (for example, in the form of the radiantly heated silicon baseplate) in the all-silicon reactor chamber of U.S. application Ser. No. 08/597,577 referred to above, then the walls of the all-silicon reactor are operated in a xe2x80x9clight depositionxe2x80x9d mode rather than in an etch mode so as to not consume (or at least to reduce the rate of consumption of) the silicon side wall or skirt and the silicon-ceiling. Thus, what is chiefly consumed is the inexpensive and quickly replaceable silicon base plate. This is best accomplished by reducing the temperature of the silicon side wall or skirt and silicon ceiling and reducing or eliminating the RF bias applied thereto (e.g, by grounding the wall, skirt and/or ceiling). Preferably, the temperature of the silicon wall and silicon ceiling and the RF bias thereon is reduced to a point at which consumption thereof is minimized by permitting a light polymer deposition thereon, but not beyond a point at which polymer deposition thereon becomes dense and difficult to remove. Such a lightly deposited polymer can be quickly and easily removed by a plasma clean step. This preserves a major advantage of the all-silicon reactor chamber in avoiding or minimizing the necessity of frequent chamber cleanings, as described in the above-referenced application. Alternatively, but not preferably, a xe2x80x9cheavy deposition modexe2x80x9d may be selected which permits a heavy polymer deposition onto the silicon wall and ceiling.
In accordance with another embodiment of the invention, separately expendable silicon pieces are disposed at different radial locations relative to the wafer being processed in order to enable independent control of etch selectivity over different radial portions of the wafer. This embodiment may be combined with the features of separately controllable inductors at respective radial locations and separately controllable electrodes at respective radial locations disclosed in co-pending U.S. application Ser. No. 08/597,577 referred to above.
It is a further discovery of the invention that the polymer-hardening precursor function fulfilled by the silicon piece requiring the elevated temperature described above does not imply a concomitantly elevated consumption rate. Where the silicon piece is not an expendable item in the reactor (or even if it is), its rate of consumption can be reduced to save cost while preserving its polymer-hardening function above the polymerization temperature. This is accomplished by reducing the RF bias applied to the silicon piece while further increasing the temperature thereof to compensate for the decrease in RF bias and thereby maintain its polymer-hardening participation in the process. In accordance with one implementation, the RF bias thereon can be decreased four-fold for a dramatic decrease in consumption rate while increasing the silicon piece temperature by only about 25% to maintain the polymer-hardening function. Preferably, the temperature is increased until the applied RF bias can be eliminated entirely.
While the preferred material employed in the various embodiments of the polymer-hardening precursor referred to above is silicon, any other suitable material whose contribution to the polymer hardness is achieved by heating a piece of it in the reactor may be employed in the foregoing embodiments. In addition to silicon, other suitable polymer-hardening precursor materials include silicon carbide, carbon and silicon nitride. Thus, the invention is more generally directed to heating a polymer-hardening precursor material of the type including silicon to at least a higher temperature range (above the polymerization temperature).